Tamora Pierce's books tends to be in the juvenile or young adult sections of the bookstores, but are great books to read. Mercedes Lackey is actually one of my favorite writers, my wife and I met in a fan group for that author, so definitely a selling point for me. In fact I do believe out of our 8 person (4 couple) gaming group, every one of us is a fan or had at least read several of Mercedes lackey's books. Can't say I've ever read Diane Duane but I may go back and look.
Note..many of Mercedes Lackey's books are considered young adult novels, although some deal with homosexuality, rape, incest, and other well not normally considered juvenile topics.
Should make for some interesting sourcebook.